1. Field of the Invention
The present invention relates to a variable-gain amplifier with high linearity as to gain variations.
2. Discussion of the Related Art
In the field of analog electronics, it is known to use variable-gain amplifiers when signals of very different amplitudes on particularly wide frequency bands are to be processed. In these cases, variable-gain amplifiers enable adapting the amplitude of the output signal to the specific working conditions, preventing saturation of the downstream-connected stage and without degrading the linearity of the signal.
Variable-gain amplifiers of the type designated by the reference number 1 in FIG. 1, are known. They include an attenuating network 2, a plurality of transconductance stages 3, a constant-gain amplifier 5, and a gain-control stage 6, which is associated to the transconductance stages 3. The attenuating network 2 is of the ladder resistive type and receives an input voltage VIN on inputs 2a, 2b. The transconductance stages 3 have first inputs each connected to a respective node 2c of the attenuating network 2 and second inputs connected to the output of the constant-gain amplifier 5 through a feedback line 7. The outputs of the transconductance stages 3 are connected to an input of the constant-gain amplifier 5. The gain-control stage 6 supplies a total constant biasing current IB, which is split among the transconductance stages 3 in a proportion indicated by a control voltage VC. In practice, it is possible to control the weight of each transconductance stage 3 for driving the constant-gain amplifier 5. The overall gain of the variable-gain amplifier 1 is greater when one of the transconductance stages 3 closest to the inputs 2a, 2b of the attenuating network 2, receiving the least attenuated input voltage VIN, prevails, and smaller when one of the transconductance stages 3 furthest from the inputs 2a, 2b prevails. The overall gain of the variable-gain amplifier 1 can thus be controlled over a wide range of values.
Known variable-gain amplifiers have, however, some drawbacks, in particular because the conditions of biasing and thus the transconductance of the individual transconductance stages vary. Even though it is possible to obtain a good linearity between the control voltage and the overall gain (designated, respectively, by VC and A in the plot of FIG. 2a), however, the harmonic distortion, for example, the third-order one, disadvantageously has an irregular pattern as the control voltage varies. In this connection, see the plot of FIG. 2b, where the third-order harmonic distortion is designated by HD3.